1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a cross-linked thermoplastic resin foam, and more specifically, to a method and an apparatus for stably continuously manufacturing a thick cross-linked thermoplastic resin foam having a smooth surface and fine cells with uniform diameters.
2. Description of the Related Art
Conventionally known are various methods for manufacturing foams by the use of a resin composition which contains a thermoplastic resin, a foaming agent, and a cross-linking agent, as essential ingredients. According to one of these methods, for example, the resin composition is molded into a predetermined shape, such as a sheet, and the resulting molded product is heat-treated under atmospheric pressure, first to decompose the cross-linking agent therein, thereby promoting cross-linking of the thermoplastic resin, and then to decompose the foaming agent, thereby expanding the cross-linked product. In this method, however, the cross-linking and forming are effected in the atmospheric-pressure, high-temperature atmosphere, so that the cells of the resulting foam are coarse and uneven in diameter. Moreover, the foam is deteriorated and made poorer in appearance due to oxidation of the resin composition, and it is hard to obtain a thick foam.
There have conventionally been proposed methods in which a closed pressing machine or die is loaded with such a resin composition as aforesaid, and the composition is heated and expanded under pressure (refer to Published Examined Japanese Patent Applications Nos. 45-40436, 46-2900 and 45-29381, and U.S. Pat. No. 4,671,910). According to these methods, in contrast with the method using the atmospheric-pressure, high-temperature atmosphere, a thick foam having fine uniform cells can be securely obtained. In these methods, however, the resin composition, which consists mainly of the thermoplastic resin compounded with cross-linking agent, and foaming agent, is cross-linked and expanded by being heated under pressure by means of the pressing machine or die. Originally based on the batch system, therefore, these methods can hardly be applied to series production. Thus, the productivity cannot be improved, and it is difficult to manufacture elongated foams due to restrictions on the shape of the pressing machine or die.
Some improved methods have been proposed to solve these problems of the batch system (refer to Published Examined Japanese Patent Application No. 60-51416, U.S. Pat. No. 4,552,708, and Published Unexamined Japanese Patent Application No. 60-11329). In the methods disclosed in Published Examined Japanese Patent Application No. 60-51416 and U.S. Pat. No. 4,552,708, the resin composition is continuously extruded from an extruder into an elongated landed die, which is connected to the extruder, so that a cross-linking reaction is advanced in the first-half stage of the elongated landed die, and gas produced by pyrolytically decomposing the foaming agent is retained in the molten resin composition by dissolution or the like in the second-half stage. Finally, the resin composition is continuously extruded into the atmosphere to be swollen. In order to retain the produced gas in the molten resin composition, a back pressure is applied to the resin composition by means of a throttle die which is located near the outlet of the landed die.
In the method disclosed in Published Unexamined Japanese Patent Application No. 60-11329, all processes, including complete cross-linking and complete decomposition of the foaming agent for the completion of expansion, are executed in an elongated landed die connected to an extruder. According to this method, therefore, the cross-linking and foaming can be finished before a cross-linked thermoplastic resin foam as a final product is continuously obtained from the landed die.
In the case of the methods disclosed in U.S. Pat. No. 4,552,708 and Published Unexamined Japanese Patent Applications No. 56-52548, No. 59-1232, No. 59-169825, No. 60-11329, No. 60-110424, No. 60-112418, No. 61-127324, No. 62-211122, No. 63-251437, No. 63-2514376, and No. 63-254142, a problem arises in that large-sized, complicated equipment is required to maintain high pressure. Further, the land die used must be significantly long because all necessary reactions must take place therein.
In the methods and apparatuses disclosed in U.S. Pat. No. 4,552,708 and Published Unexamined Japanese Patent Applications No. 59-169825, No. 60-11329, No. 61-127324, No. 62-211122, No. 62-211123, No. 63-2514376, No. 63-251437, and No. 63-254142, the cross-sectional area of the long land die is gradually increased toward the downstream end, to thereby gradually reduce the internal pressure of the die such that the volumetric expansion resulting from the expansion of the resin composition is permitted. Consequently, the shape of the long land die becomes more complicated toward the downstream end.
Due to a wide application of cross-linked foams, foams of various types, thick, thin, soft, or hard, are required and also there is a demand for a wide range of expansion ratios. It is therefore very useful to provide a method of manufacturing a cross-linked foam which method can flexibly meet various requirements. In this regard, in the case of the methods disclosed in U.S. Pat. No. 4,552,708 and Published Unexamined Japanese Patent Applications No. 56-52548, No. 59-1232, No. 59-169825, No. 60-11329, No. 60-110424, No. 60-112418, No. 61-127324, No. 62-211122, No. 63-251437, No. 63-2514376, and No. 63-254142, when the thickness or expansion ratio of cross-linked foams to be produced must be changed, the long land die in use must be replaced with another die having a different shape, followed by attachment of the die to the extruder. Further, in these methods, the resin composition in the die is passed through the throttle die while it is in a molten state or its temperature is higher than the softening point thereof. Since the resin composition which is in a molten state or at a temperature higher than the softening point and thus the flowing state of which is liable to be disturbed is forced into a narrow passage having a small cross-sectional area, a uniform flow of the resin composition is disturbed due to a strong shearing force acting thereon or stagnation occurs at the throttle die, even if lubricating oil is supplied to the inner surface of the die, thus making the manufacture of foams difficult.
The method disclosed in U.S. Pat. No. 4,552,708 is capable of continuous production of a foam having a fine cell structure. However, the foam obtained at the outlet of the die is a final product and gases and air account for almost the entire volume of the final product. Thus, the obtained foam, of which the volume is very large in comparison with the weight, must be transported, which leads to high transportation cost.
Disclosed in Published Unexamined Japanese Patent Application No. 58-134719, on the other hand, is a method for manufacturing a cross-linked thermoplastic resin foam, in which a polyolefin resin composition containing a cross-linking agent and a foaming agent is heated under a pressure of 1 to 10 kg/cm.sup.2 to decompose parts of the cross-linking and foaming agents, thereby forming cell nuclei, and is then heated under the atmospheric pressure to decompose the remaining part of the foaming agent. This manufacturing method is based on technical concepts previously disclosed in Published Examined Japanese Patent Applications Nos. 45-40436, 46-2900, etc. According to the method disclosed in Japanese Patent Application No. 45-40436, for example, parts of the cross-linking agent and the foaming agent are decomposed by heating under a pressure of 5 to 50 kg/cm.sup.2 to form cell nuclei.
In Published Unexamined Japanese Patent Application No. 58-134719, moreover, there is a description of continuous formation of cell nuclei. Described in these patent applications, however, are only the so-called batch system means as specific means for pressurizing the resin composition and the following methods as means for continuous pressurization. The batch system means include pressurization of the resin composition by means of compressed gas, autoclave, pressing machine, pressure-heating oven, etc. The continuous pressurization means include a method in which the resin composition is sandwiched between two parallel plates to be pressed thereby from both sides, and a method (load-curing) in which the resin composition is wound around a roll face and pressed from the opposite side by means of a belt. In both these continuous pressurization means, moreover, a gas pressure or mechanical force is externally applied to the resin composition to pressurize it. Thus, these pressurization means require a very special complicated pressurization mechanism. Since the resin composition is melted by heating when it is pressurized, moreover, the pressurization is believed to be very difficult. If the melted resin composition is pressed from both sides, for example, it collapses in its thickness direction, so that it is hard to maintain a fixed thickness.
In order to solve these problems, the inventors hereof developed earlier a method in which endless belts are arranged individually on the upper and lower inside surfaces of a die having a cooling region immediately following a heating region, and a resin composition is supported by means of the belts when it is continuously moved in the die. This method has already been proposed in Published Unexamined Japanese Patent Application No. 64-24728. According to this method, partial forms having cell nuclei therein can be manufactured in series. During the manufacture of the foams, however, the endless belts moving in the die are always pressed against the upper and lower inside surfaces of the die under a pressure of 11 to 50 kg/cm.sup.2, so that the die surfaces and the belts wear out severely at their sliding contact portions. In consequence, the working life of the die and the endless belts is shortened, and the belts require a large-scale drive unit, inevitably entailing increased equipment cost.
Based on a discovery that decomposing a foaming agent in a closed space, such as a die, filled with a resin composition produces a high pressure of 1,000 kg/cm.sup.2 or more, the inventors hereof developed a method in which cell nuclei are formed in the resin composition by utilizing this spontaneous pressure, without externally pressuring the composition by separate means. This method has already been proposed in Published Unexamined Japanese Patent Application No. 2-283421. According to this method, a choke bar is disposed at the outlet of a heating region in a die, and a damping force is applied to a partially foamed product in the heating region by controlling the cross-sectional area of the choke bar. By doing this, a pressure produced by the decomposition of a foaming agent in the heating region is increased to a desired level, and a cross-linking agent and the parts of foaming agent are decomposed in this state.
In the case of this method, the internal pressure of the die is increased and maintained by operating the choke bar to change the sectional area of the partially foamed product passing the choke bar, and the choke bar is arranged at each of the boundaries between the molding region and the heating region and between the heating region and the cooling region. Thus, when the partially foamed product passes the individual choke bars, it is still at high temperature and soft, and therefore, when the passage is narrowed by the choke bars, the flowing state of the partially foamed product is disturbed or stagnation occurs at the choke bars. Accordingly, the partially foamed product extruded from the die may be bent or its surface may be cracked, thus causing a difficulty in the manufacture of foams.